In general, a steel buoy is used as a mooring buoy which is used for placing and removing mooring rope of an oil drilling rig or the like in an offshore area (for example, refer to Patent Document 1). However, because the oil drilling rig is moved with a change of places for drilling, at every time of the movement, construction for placing and removing an anchor, mooring rope and the mooring buoy is repeatedly carried out. Additionally, at every time of the movement, the above steel buoys are piled on a working boat and the buoys are unloaded at the next place for drilling.
However, because the steel buoy has stiffness while being a heavy load, there have been a lot of problems that the working boat, and an object on the working boat are damaged when the steal buoy is in contact with them and that the buoys occupy a space on the working boat.
Additionally, because the steel buoy does not have shock-absorbing properties, it have often happed that, when the steel buoy collides with another object, the buoy itself becomes recessed and damaged.
Moreover, when the steel buoy is used in a water zone with a depth deeper than a specified depth, by mistake, the buoy sometimes crushes due to a hydraulic pressure. Once the steel buoy has thus crushed, collection of the steel buoy becomes difficult. In addition, even if the steel buoy is collected, it is impossible to reuse the steel buoy because an original form thereof cannot be restored.
Furthermore, because steel which is material for the steel buoy is corrosive, while the buoy is being placed in the water, the buoy should be disembarked at regular intervals to be treated with maintenance for corrosion prevention, or to be replaced in some situations.
[Patent Document 1] Japanese patent application Kokai publication No. Hei 10-310095